Labour are 22 points ahead in the Feltham & Heston by-election, according to my latest poll. With a 52% vote share in the week before polling day, Seema Malhotra seems to be on course for what looks like a convincing victory.

But these numbers, with Labour up 8 points on their general election vote share, and Conservative Mark Bowen down 4 points on 30%, represent just a 6-point swing from the Tories to Labour. That would be a big result in a general election. But in a by-election – an easy opportunity for a cost-free anti-government protest vote at a time of economic gloom – Labour might have been expecting to do better. The swing is only a third of what the Conservatives achieved in Crewe & Nantwich or Norwich North in the last parliament (swings that we did not come close to matching across the board in May 2010).

Not surprisingly in a Labour constituency, most people think the coalition government is doing badly overall rather than well. Even here, though, David Cameron is seen as the best available Prime Minister, if only by a slim margin. A third of Liberal Democrat voters, and even one in seven who say they will vote Labour, see him as the best man for the job.

As well as the relative merits of the leaders, the parties' approaches to the economy once again help to explain the resilience of the government's position and the lack of a Labour breakthrough. While Labour enjoy big leads when it comes to sharing voters' values and being on the side of ordinary people, a majority of the constituency's voters believe the Conservatives are willing to take tough decisions for the long term.

Accordingly, they decisively favour a description of Plan A: "borrowing more will make matters worse…we have to bring the debt and the deficit under control even if it has some painful effects for the economy in the short term" over Plan B: "the government's spending cuts and tax rises are hurting the economy. It should cut taxes, and/or cut spending less fast, even if that means we go on borrowing more for longer". (Yes, when describing their own policy Labour usually leave out the part about borrowing more for longer – but as my research on the economy last month showed, voters have already clocked that more borrowing inevitably follows from arguing that cuts are too far and too fast).

That is not to say people in Feltham & Heston are optimistic about the economy. Nearly two thirds think the economic situation in three or four years' time will be as bad as it is now, or worse. Again, though, this brings no comfort for Labour. Asked who they most trust to manage the economy given the challenges it faces, 47% chose Cameron, Nick Clegg and George Osborne, and only 35% Ed Miliband and Ed Balls.

Looking further ahead, the poll confirms that Boris Johnson is running well ahead of the Conservatives in London. Even in what is about to become a safe Labour seat, Boris is running neck and neck with Ken Livingstone. A third of those who say they will vote Liberal Democrat at the by-election on Thursday, and a quarter of those planning to vote Labour, see themselves backing Boris five months from now.

1,503 adults in the Feltham & Heston constituency were interviewed by telephone between 1 and 6 December 2011. Data have been weighted to be representative of all adults living in the constituency. For full data tables go to lordashcroft.com.